1. Field of the Invention
The invention relates generally to micro electromechanical systems (MEMS). In particular, the invention relates to a method of fabricating an array of tiltable MEMS micro mirrors.
2. Background Art
The technology of micro electromechanical systems (MEMS) has received increasing attention in recent years as it has demonstrated that mechanical systems and their electrical actuators can be effectively and economically combined on a microscopic scale much smaller than that typical in traditional mechanical systems assembled from machined parts. MEMS devices are typically fabricated from silicon wafers or other silicon-based substrates using techniques well developed in the integrated circuit industry. These techniques include photolithography, etching, sputtering, and chemical vapor deposition, for all of which 1 μm resolution is readily available.
A MEMS device is advantageously implemented as an optical switch in an optical communication system based upon optical fiber communications links, as has been described by Smith et al. in U.S. patent application Ser. No. 09/957,312, filed Sep. 20, 2001 and now issued as U.S. Pat. No. 6,798,941, incorporated herein by reference in its entirety. Solgaard et al. disclose an earlier version of such a device in U.S. Pat. No. 6,097,859. Such a MEMS device 10 appropriate for the present invention is illustrated schematically in plan view in FIG. 1. It may include an array of tiltable micromirrors 12 formed in a substrate 14 and arranged in a 2-dimensional array. A gimbal structure allows each of a large number of mirrors 12 to tilt in one direction about an axis of a first pair of torsion beams 16 integrally joining the mirror 12 to substrate 14 and to tilt in an orthogonal direction about an orthogonal axis of a second pair of similar torsion beams 18. The mirrors 12 are separately and independently tiltable by electrical actuators included within the MEMS. Typically, opposed electrodes in the mirror 12 and substrate 14 allow independent electrostatic actuation about the respective axes for the respective mirrors 12.
In a wavelength division multiplexing (WDM) communication systems in which optical carriers of different wavelengths are impressed on the optical fiber link and the switch selectively connects wavelength-separated signals between the fibers, one direction in the array may correspond to wavelength and the other to the fiber. In this case, there may be 80 or more WDM wavelengths and 6 or more input and output fibers. Further, the number of mirrors in the fiber direction is often doubled since two fibers may be linked via two mirrors through a coupling mirror. That is, there may be up to a thousand or more of such tiltable mirrors 12 formed in a single substrate 10 having dimensions of only a few centimeters. One baseline design includes 80×12=960 mirrors in a two-dimensional array spaced at 470 μm in the wavelength direction and 750 μm in the fiber direction. A similar micro mirror array may be used in white-light communication systems although the number of mirrors corresponds generally to the number of fiber so that the number, while significant, is typically much less than in a WDM system.
MEMS arrays on the level of integration contemplated here present challenges in integrating the MEMS structures with the electrical circuitry controlling and driving the electrical actuators. While wire bonding external circuitry to the MEMS structure is feasible for a few actuators on the order of about a hundred, it becomes problematic for the large arrays described above. The wire bonding is time consuming, and the large bonding pads occupy too much space. Furthermore, if the electrical interconnects linking bonding pads to the actuators are formed on the same surface as the mirrors, the fill factor of total mirror area to chip area further decreases.
Garverick et al. have proposed a multiplexed electrostatic actuator system in U.S. patent application Ser. No. 09/884,676, filed Jun. 19, 2001, now issued as U.S. Pat. No. 6,543,286, and published as PCT Application WO 02/060045 A2, incorporated herein by reference in their entireties. The multiplexing allows relatively few, for example, less than 30, electrical lines to control the entire micro mirror array. However, each actuator still requires its own high-voltage drive line. Furthermore, the multiplexing requires the overall system to also include both low-voltage digital control circuitry of moderate complexity and some high-voltage circuitry to directly drive the electrostatic electrodes.
In U.S. patent application, Ser. No. 10/120,869, filed Apr. 11, 2002 , now issued as U.S. Pat. No. 6,694,073, and incorporated herein by reference in its entirety and in International Publication No. WO 02/1084372 A2, Golub et al. have disclosed a free-space optical system that may be based on such an array of MEMS mirrors. The optical design is simplified if the overall extent of the mirror array is minimized while the area of each mirror is maximized, that is, a MEMS mirror array with a high fill factor on the MEMS substrate.
The design of the MEMS micro mirrors can be improved. The mirror itself is subject to countervailing requirements. It should be relatively stiff so that it is not subject to warping and flexing. Mirror deformation should be controlled to less than λ/8, which corresponds to approximately 0.19 μm for the wavelengths of interest. On the other hand, the tiltable mirror should be relatively light to increase its resonant frequency to avoid switching transients. Furthermore, the torsion beams should be made flexible so that a small actuator voltage can produce a large mirror tilt, which either simplifies the electronics or the optics or enables coupling between a larger number of fibers. Tilts of ±4° about the center orientation are desired. Long torsion bars are difficult to fabricate and result in fragile structures. Further, the typical MEMS torsion beam is etched from crystalline silicon. Reducing the cross section of the silicon increases the probability of fracture of the crystalline material. An optical switch fielded in a commercial telecommunications network must be highly reliable, and the large number of mirrors in the array exacerbates the failure problem.
One way to effect electrostatic actuation of the tiltable mirrors places electrodes in the substrate under the mirror and separated from it by a gap to allow the mirror to tilt into the gap. Fabricating such buried electrodes is not a straightforward process.